Processes for recovering silane from heavy-ends separation operations

ABSTRACT

Processes and systems for purifying silane-containing streams are disclosed with relatively less silane being lost in impurity streams by use of distillation and/or condensation operations.

BACKGROUND

The field of the disclosure relates to purification of silane-containingstreams and, particularly, to methods for purifying silane withrelatively less silane being lost in impurity streams by use ofdistillation and/or condensation operations.

Silane is a versatile compound that has many industrial uses. In thesemiconductor industry, silane may be utilized for deposition of anepitaxial silicon layer on semiconductor wafers and for production ofpolycrystalline silicon. Polycrystalline silicon is a vital raw materialused to produce many commercial products including, for example,integrated circuits and photovoltaic (i.e., solar) cells that may beproduced by thermal decomposition of silane onto silicon particles in afluidized bed reactor.

Silane may be produced by reacting silicon tetrafluoride with an alkalior alkaline earth metal aluminum hydride such as sodium aluminumtetrahydride as disclosed in U.S. Pat. No. 4,632,816, which isincorporated herein by reference for all relevant and consistentpurposes. Silane may alternatively be produced by the so-called “UnionCarbide Process” in which metallurgical-grade silicon is reacted withhydrogen and silicon tetrachloride to produce trichlorosilane asdescribed by Müller et al. in “Development and Economic Evaluation of aReactive Distillation Process for Silane Production,” Distillation andAdsorption: Integrated Processes, 2002, which is incorporated herein byreference for all relevant and consistent purposes. The trichlorosilaneis subsequently taken through a series of disproportionation anddistillation steps to produce a silane end-product.

After silane is produced, it is conventionally taken through apurification process to remove impurities prior to use (e.g., prior toepitaxial layer production or polycrystalline silicon production).Examples of impurities that may be present in the silane-containingprocess streams include, for example, nitrogen, methane, hydrogen,ethane, ethylene, ethyl-silane, diethyl silane, toluene, dimethoxyethaneand combinations of these impurities. Examples of such purificationprocesses include those disclosed in U.S. Pat. Nos. 5,206,004; 4,554,141and 5,211,931, each of which is incorporated herein by reference for allrelevant and consistent purposes. Such conventional processes mayadequately purify silane-containing process streams; however, they arecharacterized by relatively high rates of unrecoverable silane.

A continuing need therefore exists for processes for purifyingsilane-containing process streams that achieve relatively high silanepurity and a relatively high rate of silane recovery. A need also existsfor systems for such processes.

SUMMARY

One aspect of the present disclosure is directed to a process forpurifying a silane-containing stream. The stream contains silane and oneor more compounds having a boiling point greater than silane. Theprocess includes introducing the silane-containing stream to aheavy-ends distillation column to produce a silane-enriched overheadfraction and a silane-depleted bottoms fraction relative to thesilane-containing stream. The silane-depleted bottoms fraction containssilane and is enriched in one or more compounds having a boiling pointgreater than silane relative to the silane-containing stream. Thesilane-depleted bottoms fraction is introduced into a silane-recoveryseparation unit to produce a silane-enriched fraction and asilane-depleted fraction relative to the silane-depleted bottomsfraction produced from the heavy-ends distillation column.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a system for recovering silane from theoverhead fraction of a light-ends distillation column according to afirst embodiment of the present disclosure;

FIG. 2 is a schematic of a silane-recovery separation unit forrecovering silane from the overhead fraction of a light-endsdistillation column according to a first embodiment of the presentdisclosure;

FIG. 3 is a schematic of a system for recovering silane from theoverhead fraction of a light-end distillation column according to asecond embodiment of the present disclosure;

FIG. 4 is a schematic of a system for recovering silane from theoverhead fraction of a light-ends distillation column according to athird embodiment of the present disclosure;

FIG. 5 is a schematic of a system for recovering silane from the bottomsfraction of a heavy-ends distillation column according to a firstembodiment of the present disclosure;

FIG. 6 is a schematic of a system for recovering silane from the bottomsfraction of a heavy-ends distillation column according to a secondembodiment of the present disclosure;

FIG. 7 is a schematic of a system for recovering silane from the bottomsfraction of a heavy-ends distillation column according to a thirdembodiment of the present disclosure;

FIG. 8 is a schematic of a system for recovering silane from the bottomsfraction of a heavy-ends distillation column according to a fourthembodiment of the present disclosure; and

FIG. 9 is a schematic of a system for recovering silane from theoverhead fraction of a light-ends distillation column and from thebottoms fraction of a heavy-ends distillation column according to afirst embodiment of the present disclosure.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

In accordance with various embodiments of the present disclosure,processes for purifying silane that involve distillation operations andwhich achieve high purity and high silane recovery relative toconventional processes are herein disclosed. Further, purificationsystems for carrying out such processes are provided. Embodiments ofsuch processes and systems may include recovery of silane from processstreams separated from the silane-containing stream (e.g., processstreams that include a relatively higher amount of compounds with aboiling point less than silane and/or streams that include a relativelyhigher amount of compounds with a boiling point greater than silanerelative to the silane-containing stream which is purified). Silane maybe removed from these impurity streams by use of distillation orcondensation operations as discussed further below.

Silane-Containing Streams that may be Purified

The silane-containing streams that may be purified according toembodiments of the present disclosure may contain a wide variety ofimpurities and may contain those impurities in a wide range of amountswithout departing from the scope of the present disclosure. Thesilane-containing stream may have been subjected to one or morepurification steps prior to performing the purification process ofembodiments of the present disclosure (e.g., removal of an amount ofimpurities with a boiling point less than silane or a boiling pointgreater than silane). The amount of silane in the silane-containingstream prior to being subjected to the purification processes of thepresent disclosure may be at least about 10 wt %, at least about 30 wt%, at least about 50 wt %, at least about 70 wt %, at least about 85 wt%, at least about 95 wt % or even at least about 99 wt % (e.g., fromabout 50 wt % to about 99 wt % or from about 70 wt % to about 99 wt %).The silane-containing stream may be substantially free (e.g., containless than about 0.1 mol % or less than about 0.01 mol%) of alkali oralkaline earth-metal silanes and, alternatively or in addition, may besubstantially free of trichlorosilane, tetrachlorosilane andtetrafluorosilane. In some embodiments, these compounds aresubstantially removed from the silane-containing stream prior toperforming the processes of the present disclosure.

The silane-containing stream that is subjected to the processes of thepresent disclosure may contain one or more impurities with a boilingpoint greater than silane and/or one or more impurities with a boilingpoint less than silane. In this regard, the boiling point of silane isabout −112° C. Impurities that have a boiling point greater than silaneinclude, for example, ethane, ethylene, ethyl-silane, diethyl silane,toluene, and dimethoxyethane. Impurities that have a boiling point lessthan silane include, for example, hydrogen, nitrogen, and methane.

The starting silane-containing stream may be a gas or may be a liquidwithout departing from the scope of the present disclosure. In thisregard, the silane-containing stream may be within any range oftemperatures and pressures including, but not limited to, pressure andtemperatures typical of silane production and/or processing.

Processes for Purifying Silane that Include Recovery from Light-endsStreams

Referring now to FIG. 1 in which a process for purifying silane is shownschematically in accordance with embodiments of the present disclosure,a silane-containing stream 3 is introduced into a light-endsdistillation column 5 to produce a silane-enriched bottoms fraction 6and silane-depleted overhead fraction 11 relative to thesilane-containing stream 3. The silane-containing stream 3 containssilane and one or more compounds having a boiling point less than silane(e.g., hydrogen, nitrogen and/or methane). The silane-containing stream3 may be condensed and compressed prior to introduction into thelight-ends distillation column 5. As used herein, “light-endsdistillation column” refers to a distillation column in which at leastabout 50 wt % of the compounds with a boiling point less than silane areseparated from the light-ends distillation column feed stream (or feedstreams when more than one feed stream is used). Generally, theseseparated compounds are removed from the distillation column 5 in thesilane-depleted overhead fraction 11. In some embodiments of the presentdisclosure, the overhead fraction 11 contains at least about 50 wt % ofthe compounds with a boiling point less than silane in thesilane-containing stream 3 and, in other embodiments, at least about 60wt %, at least about 75 wt %, at least about 90 wt % or even at leastabout 95 wt % of the compounds with a boiling point less than silane inthe silane-containing stream 3. The silane-depleted overhead fraction 11may contain at least about 40 wt % silane and, in other embodiments,contains at least about 50 wt %, at least about 60 wt %, at least about70 wt % or at least about 75 wt % silane.

Generally, the overhead fraction 11 is enriched in compounds with aboiling point less than silane. In embodiments wherein thesilane-containing stream 3 contains compounds with a boiling pointgreater than silane, the bottoms fraction 6 is enriched in thesecompounds and the overhead fraction 11 is depleted in these compounds.

The light-ends distillation column 5 may be operated at temperatures andpressures suitable for separating compounds having a boiling point lessthan silane from silane as appreciated by those of skill in the art. Forexample, the light-ends distillation column 5 may be operated at anoverhead fraction 11 temperature of from about −130° C. to about −50° C.or from about −110° C. to about −70° C. and an overhead fractionpressure of from about 1100 kPa to about 2500 kPa (about 160 psia toabout 360 psia) or from about 1400 kPa to about 2200 kPa (about 200 psiato about 320 psia). The bottoms fraction 6 temperature may be from about−50° C. to about −120° C.

Upon discharge from the light-ends distillation column 5, thesilane-depleted overhead fraction 11 conventionally is neutralized anddisposed as a waste stream. In accordance with the present disclosure,it has been found that the overhead fraction 11 may be processed torecover a significant amount of silane. In some embodiments of thepresent disclosure, the silane-depleted overhead fraction 11 producedfrom the light-ends distillation column 5 is introduced into asilane-recovery separation unit 30 to produce a silane-enriched fraction32 and a silane-depleted fraction 35 relative to the silane-depletedoverhead fraction 11 produced from the light-ends distillation column 5.The silane-enriched fraction 32 may be recovered for use by recyclingthe silane-enriched fraction back to the light-ends distillation column5.

The silane-recovery separation unit 30 may be any unit (or even morethan one unit) suitable for separating silane from compounds with aboiling point less than silane. For example, the unit 30 may be one ormore distillation columns that separate the silane-depleted overheadfraction 11 produced from the light-ends distillation column 5 into asilane-enriched bottoms fraction 32 and a silane-depleted overheadfraction 35 relative to the silane-depleted overhead fraction 11.Alternatively and as shown in FIG. 2, the silane-depleted overheadfraction 11 produced from the light-ends distillation column 5 (FIG. 1)may be cooled to condense a portion of the silane therein, therebyseparating silane from the more volatile compounds having a boilingpoint less than silane. The overhead fraction 11 may be introduced intoa condenser 37 to condense silane. The silane-condensed overheadfraction 42 may be introduced into a gas-liquid separator 38 to producea silane-depleted gaseous stream 35 and a silane-enriched liquid stream51 relative to the silane-depleted overhead fraction 11 produced fromthe light-ends distillation column 5. The silane-enriched liquid stream51 contains condensed silane which may be recycled for use.

Optionally and as shown in FIG. 2, the overhead fraction 11 may be firstintroduced into a heat exchanger 36 (e.g., an interchanger) to cool andpotentially partially condense the silane-depleted overhead fraction 11produced from the light-ends distillation column 5. The overheadfraction 11 may be thermally contacted with the liquid stream 51produced from the gas-liquid separator 38 to condense a portion of thesilane in the silane-depleted overhead fraction 11 produced from thelight-ends distillation column 5.

The heat exchanger 36 may be any suitable apparatus for cooling and orcondensing silane-containing streams such as, for example, a shell andtube heat exchanger with the silane-containing gaseous stream 11 beingon the shell side and the condensed silane stream 51 being on the tubeside. The condenser 37 may also be a shell and tube exchanger with thecooled and/or partially condensed silane-containing gas stream 41 beingon the shell side and a cooling fluid (e.g., liquid nitrogen) being onthe tube side. The gas-liquid separator 38 may include vessels in whichthe pressure, temperature and/or velocity of the incoming stream 42 isreduced causing entrained liquid to separate from the gas. Thegas-liquid separator 38 may include a demister to further causeentrained liquid to aggregate into droplets for removal from the gasstream.

In this regard, it should be understood that optionally and in severalembodiments of the present disclosure, the process is performed withoutthe heat exchanger 36 and the overhead fraction 11 is introduceddirectly into the condenser 37. Regardless of whether a heat exchanger36 is used, the overhead fraction 11 may be cooled to less than about−100° C. in the condenser 37, less than about −115° C., less than about−130° C., less than about −145° C. or even less than about −160° C.(e.g., at pressures from about 1100 kPa to about 2500 kPa (about 160psia to about 360 psia) or from about 1400 kPa to about 2200 kPa (about200 psia to about 320 psia).

In embodiments wherein the heat exchanger 36 is used, the overheadfraction 11 may be cooled to less than about −50° C. in the heatexchanger or less than about −70° C., less than about −85° C. or evenless than about −100° C. (e.g., at pressures from about 1100 kPa toabout 2500 kPa (about 160 psia to about 360 psia) or from about 1400 kPato about 2000 kPa (about 200 psia to about 290 psia).

Regardless of whether a heat exchanger 36 is used with the condenser 37,the amount of silane in the silane-depleted overhead fraction 11produced from the light-ends distillation column 5 that is condensed bythe condenser 37 and, optionally, the heat exchanger 36 may be at leastabout 60% and, in other embodiments, at least about 75%, at least about85%, at least about 90% or at least about 95% of the silane in thesilane-depleted overhead fraction 11 is condensed.

The liquid stream 51 produced from the gas-liquid separator 38 may beintroduced into the heat exchanger 36 to condense a portion of thesilane in the silane-depleted overhead fraction 11. A portion of thesilane-enriched liquid stream 51 may be partially vaporized in the heatexchanger 36 and the stream may be heated to a temperature ranging from−125° C. to about 10° C. In some embodiments, the silane-enriched liquidstream 51 is fully vaporized in the heat exchanger 36.

The silane stream 32 exiting the heat exchanger 36 may optionally beheated to about ambient temperature to combine with crude silane 3(FIG. 1) which may be pressurized for introduction into the light-endsdistillation column 5. Suitable heaters (not shown) include, forexample, finned exchangers that use ambient air to heat the silanestream 32. The vaporized silane stream 32 may be recondensed andrecycled to the light-ends distillation column 5. The silane stream 32may be relatively pure and, in some embodiments, contains at least about90 wt % silane and, in other embodiments, at least about 92 wt %, atleast about 95 wt %, or at least about 97 wt % silane (e.g., from about90 wt % to 99.9 wt % or from about 95 wt % to about 99.9 wt %).Alternatively or in addition, at least about 80 wt % of the silane inthe overhead fraction 11 produced from the light-ends distillationcolumn 5 is recovered in the silane stream 32 and, in other embodiments,at least about 75 wt %, at least about 85 wt %, at least about 90 wt %,at least about 93 wt %, at least about 96 wt % or even at least about 98wt % of silane in the overhead fraction 11 is recovered in the silanestream 32 (e.g., from about 75 wt % to about 99.9 wt %, from about 85 wt% to about 99.9 wt % or from about 93 wt % to about 99.9 wt %).

The silane-depleted overhead fraction 35 produced from thesilane-recovery separation unit 30 may be treated (e.g., neutralized)and disposed of (e.g., by exhausting it to the ambient). The overheadfraction 35 may be heated by use of, for example, a natural convectionfinned heat exchanger (not shown) before disposal. In accordance withembodiments of the present disclosure, use of the silane-recoveryseparation unit 30 allows the amount of waste gas that is naturalizedand disposed of to be reduced relative to conventional processes inwhich a silane-recovery separation unit 30 is not used. For example, atleast about 60 wt % of the overhead fraction 11 may be recovered in thesilane stream 32 or, as in other embodiments, at least about 65 wt %, atleast about 70 wt % or even at least about 75 wt % of the overheadfraction 11 is recovered in the silane stream 32 (e.g., from about 65 wt% to about 90 wt % or from about 65 wt % to about 80 wt %).

The overhead fraction 35 preferably contains a minimal amount of silaneand, in some embodiments, contains less than about 10 wt % of the silanein the silane-depleted overhead fraction 11 produced from the light-endsdistillation column 5. In other embodiments, the overhead fraction 35contains less than about 8 wt %, less than about 6 wt %, or less thanabout 4 wt % of the silane in the silane-depleted overhead fraction 11produced from the light-ends distillation column 5.

In accordance with several embodiments of the present disclosure, crudesilane may also contain compounds having a boiling point greater thansilane. When the silane-containing stream 3 (FIG. 1) contains suchcompounds with a boiling point greater than silane, the silane-enrichedbottoms fraction 6 produced from the light-ends distillation column 5typically is enriched in compounds with a boiling point greater thansilane relative to the silane-containing stream 3. To separate silanefrom these relatively high boiling point compounds and as shown in FIG.3, the silane-enriched bottoms fraction 6 may be introduced into aheavy-ends distillation column 15 (which may be operated in accordancewith the parameters described below in the section entitled “Processesfor Purifying Silane that include Recovery from Heavy-ends Streams”) toproduce a silane-enriched overhead fraction 43 and a silane-depletedbottoms fraction 31 relative to the silane-enriched bottoms fraction 6produced from the light-ends distillation column 5. The silane-depletedbottoms fraction 31 is enriched in compounds having a boiling pointgreater than silane and may be treated (e.g., neutralized) and disposedof Alternatively, the silane-depleted bottoms fraction 31 may be treatedto recover silane as described below.

The light-ends distillation column 5, silane recovery separation unit 30and heavy-ends distillation column 15 may purify silane with smallamounts of silane being lost relative to conventional processes. Forexample and in accordance with some embodiments of the presentdisclosure, the sum of the amount of silane in the waste streams of theprocess of FIG. 3 (i.e., the silane-depleted gaseous stream 35 separatedin the silane-recovery separation unit 30 and the silane-depletedbottoms fraction 31 produced from the heavy-ends distillation column 15)is less than about 15 wt % of the silane in the silane-containing stream3 and, in other embodiments, less than about 11 wt %, less than about 8wt % or less than about 4 wt % of the silane in the silane-containingstream 3.

Generally, in contrast to processes in which trichlorosilane is takenthrough a series of disproportionation and distillation steps to producea silane end-product, in embodiments of the present disclosure forpurifying silane, the components of the silane-containing stream 3 donot substantially undergo any reaction until being discharged as theoverhead fraction 11 produced from the light-ends distillation column 5,as the silane-enriched overhead fraction 43 produced from the heavy-endsdistillation column 15, or the silane-depleted bottoms fraction 31produced from the heavy-ends distillation column 15.

In some embodiments, the silane-containing stream 3 contains an amountof ethylene which exits the heavy-ends distillation column 15 in thesilane-enriched overhead fraction 43. The ethylene and silane may beseparated by use of an adsorber (e.g., molecular sieve) and/or anethylsilane distillation column as further described below.

In certain other embodiments of the present disclosure, thesilane-containing stream 3 may be formed by having compounds with aboiling point greater than silane removed prior to introduction into thelight-ends distillation column 5. As shown in FIG. 4, a feed stream 7containing silane, one or more compounds having a boiling point greaterthan silane and one or more compounds having a boiling point less thansilane is introduced into a heavy-ends distillation column 15′ toproduce an overhead fraction 3′ and a bottoms fraction 31′. The overheadfraction 3′ is enriched in silane and enriched in compounds having aboiling point less than silane relative to the feed stream 7. A portionof the overhead fraction 3′ forms the silane-containing stream 3 that isfed to the light-ends distillation column 5. The bottoms fraction 31′ isdepleted in silane and enriched in compounds having a boiling pointgreater than silane and may be neutralized and disposed of or may betreated to recover silane as described below.

In this regard, the process of FIG. 3 in which light-ends distillation 5is performed before heavy-ends distillation 15 may be preferred to theprocess of FIG. 4 in which heavy-ends distillation 15′ is performedprior to light-ends distillation 5 as the lower pressures of thedown-stream distillation (i.e., the heavy-ends distillation of FIG. 3and the light-ends distillation of FIG. 4) are better suited forrecovery in heavy-ends distillation operations as compared to light-endsdistillation operations.

Processes for Purifying Silane that Include Recovery from Heavy-endsStreams

Referring now to FIG. 5 in which another process for purifying silane isshown schematically in accordance with embodiments of the presentdisclosure, a silane-containing stream 7 is introduced into a heavy-endsdistillation column 15 to produce a silane-enriched overhead fraction 43and a silane-depleted bottoms fraction 31 relative to thesilane-containing stream 7. The silane-containing stream 7 containssilane and one or more compounds having a boiling point greater thansilane (e.g., ethane, ethylene, ethyl-silane, diethyl silane, toluene,and/or dimethoxyethane). The silane-containing stream 7 may be condensedand compressed prior to introduction into the heavy-ends distillationcolumn 15. As used herein, “heavy-ends distillation column” refers to adistillation column in which at least about 50 wt % to the compoundswith a boiling point greater than silane are separated from theheavy-ends distillation column feed stream (or feed streams when morethan one feed streams are used). Generally, these separated compoundsare removed from the heavy-ends distillation column 15 insilane-depleted bottoms fraction 31. The silane-depleted bottomsfraction 31 includes an amount of silane and is enriched in one or morecompounds having a boiling point greater than silane relative to thesilane-containing stream 7. In some embodiments of the presentdisclosure, the bottoms fraction 31 contains at least about 50 wt % ofthe compounds with a boiling point greater than silane in thesilane-containing stream 7 and, in other embodiments, at least about 60wt %, at least about 75 wt %, at least about 90 wt % or even at leastabout 95 wt % of the compounds with a boiling point greater than silanein the silane-containing stream 7. The silane-depleted bottoms fraction31 may include at least about 20 wt % silane and, in other embodiments,contains at least about 40 wt %, at least about 60 wt %, at least about80 wt %, at least about 90 wt % or at least about 93 wt % silane.

Generally, the bottoms fraction 31 is enriched in compounds with aboiling point greater than silane. In embodiments wherein thesilane-containing stream 7 contains compounds with a boiling point lessthan silane, the overhead fraction 43 is enriched in these compounds andthe bottoms fraction 31 is depleted in these compounds.

The heavy-ends distillation column 15 may be operated at temperaturesand pressures suitable for separating compounds having a boiling pointgreater than silane from silane as appreciated by those of skill in theart. In this regard, relatively higher bottoms fraction 31 temperaturesgenerally reduce the amount of silane in the bottoms fraction 31 butincrease the amount of impurities in the silane-enriched overheadfraction 43. Accordingly, the column 15 may be operated belowtemperatures at which the amount of impurities in the overhead fraction43 is unacceptable to purification operations. For example, theheavy-ends distillation column 15 may be operated at a bottoms fraction31 temperature of less than about 10° C. and, in other embodiments, lessthan about 0° C., less than about −10° C., less than about −20° C., lessthan about −30° C. or even less than about −40° C. (e.g., from about−50° C. to about 10° C., from about −50° C. to about 0° C. or from about−40° C. to about 0° C.). The distillation column 15 may be operated atpressures from about 1250 kPa to about 3000 kPa (about 180 psia to about440 psia) or from about 1500 kPa to about 2500 kPa (about 220 psia toabout 360 psia).

Upon discharge from the heavy-ends distillation column 15, thesilane-depleted bottoms fraction 31 is conventionally neutralized anddisposed of as a waste stream. In accordance with the presentdisclosure, it has been found that the bottoms fraction 31 may beprocessed to recover a significant amount of silane. In some embodimentsof the present disclosure, the silane-depleted bottoms fraction 31produced from the heavy-ends distillation column 15 is introduced into asilane-recovery separation unit 10 to produce a silane-enriched fraction13 and a silane-depleted fraction 8 relative to the silane-depletedbottoms fraction 31 produced from the heavy-ends distillation column 15.The silane-enriched fraction 13 may be recovered for use by recyclingthe silane-enriched fraction 13 back to the heavy-ends distillationcolumn 15 or, as in embodiments wherein a light-ends distillation column(not shown) is used in addition to a heavy-ends distillation column 15as described below, by recycle to the light-ends distillation column.The silane-enriched fraction 13 may be further processed and/or purifiedas further described below (e.g., removal of ethylene and itsderivatives).

The silane-recovery separation unit 10 may be any unit (or even morethan one unit) suitable for separating silane from compounds with aboiling point greater than silane. For example, the unit 10 may be oneor more distillation columns that separate the silane-depleted bottomsfraction 31 produced from the heavy-ends distillation column 15 into asilane-enriched overhead fraction 13 and a silane-depleted bottomsfraction 8 relative to the silane-depleted bottoms fraction 31 producedfrom the heavy-ends distillation column 15. In some embodiments whereinthe silane-recovery separation unit 10 is a distillation column, thecolumn may be operated such that the temperature of the silane-depletedbottoms fraction 8 is from about 10° C. to about 80° C. or from about20° C. to about 50° C. upon discharge from the silane-recoverydistillation column 10. In some embodiments wherein the silane-recoveryseparation unit 10 is a distillation column, the column may be operatedsuch that the temperature of the silane-depleted bottoms fraction 8 isfrom about 70° C. to about 130° C. or from about 85° C. to about 105° C.upon discharge from the silane-recovery distillation column 10. Thedistillation column 10 may be operated at an overhead pressure of fromabout 1500 kPa to about 2600 kPa (about 220 psia to about 380 psia) orfrom about 1700 kPa to about 2200 kPa (about 250 psia to about 320psia).

The silane-enriched overhead fraction 13 produced from thesilane-recovery separation unit 10 that is recovered for use may containa small amount of impurities; however, the overhead fraction 13 maycontain at least about 90 wt % silane and, in other embodiments, atleast about 92 wt %, at least about 95 wt %, or at least about 97 wt %silane (e.g., from about 90 wt % to about 99.9 wt % or from about 95 wt% to about 99.9 wt %). Alternatively or in addition, at least about 80wt % of the silane in the bottoms fraction 31 produced from theheavy-ends distillation column 15 is recovered in the overhead fraction13 produced from the silane-recovery separation unit 10 and, in otherembodiments, at least about 75 wt %, at least about 85 wt %, at leastabout 90 wt %, at least about 93 wt %, at least about 96 wt % or even atleast about 98 wt % of silane in the bottoms fraction 31 is recovered inthe overhead fraction 13 (e.g., from about 75 wt % to about 99.9 wt %,from about 85 wt % to about 99.9 wt % or from about 93 wt % to about99.9 wt %).

The silane-depleted bottoms fraction 8 produced from the silane-recoveryseparation unit 10 may be treated (e.g., neutralized) and disposed of byexhausting it to the ambient. In accordance with embodiments of thepresent disclosure, use of the silane-recovery separation unit 10 allowsthe amount of waste gas that is neutralized and disposed of to bereduced relative to conventional processes in which a silane-recoveryseparation unit 10 is not used. For example, at least about 30 wt % ofthe bottoms fraction 31 may be recovered in the overhead fraction 13produced from the silane-recovery separation unit 10 or, as in otherembodiments, at least about 40%, at least about 50%, at least about 60%or even at least about 70% of the bottoms fraction 31 produced from theheavy-ends distillation column 15 may be recovered in the overheadfraction 13 (e.g., from about 30% to about 90%, from about 30% to about80% or from about 40% to about 80%).

The bottoms fraction 8 preferably contains a minimal amount of silaneand, in some embodiments, contains less than about 10 wt % of the silanein the silane-depleted bottoms fraction 31 produced from the heavy-endsdistillation column 15. In other embodiments, the bottoms fraction 8contains less than about 7.5 wt %, less than about 5 wt %, less thanabout 2.5 wt % or less than about 1% of the silane in thesilane-depleted bottoms fraction 31 produced from the heavy-endsdistillation column 15.

In accordance with several embodiments of the present disclosure, crudesilane may also contain compounds having a boiling point less thansilane. When the crude silane contains such compounds with a boilingpoint less than silane, a light-ends distillation column 5 (which may beoperated in accordance with the parameters described above in thesection entitled “Processes for Purifying Silane that include Recoveryfrom Light-ends Streams”) may be used to separate these compounds. Inone or more exemplary embodiments and as shown in FIG. 6, a feed stream3 is introduced into a light-ends distillation column 5 to produce anoverhead fraction 11 and a bottoms fraction 7 that forms a portion ofthe silane-containing stream introduced into the heavy-ends distillationcolumn 15. The feed stream 3 contains silane, one or more compoundshaving a boiling point greater than silane and one or compounds having aboiling point less than silane. The bottoms fraction 7 that isintroduced into the heavy-ends distillation column 15 is enriched insilane and depleted in compounds having a boiling point less thansilane. The overhead fraction 11 is depleted in silane and enriched incompounds having a boiling point less than silane. The overhead fraction11 may be treated (e.g., neutralized) and disposed of. Alternatively,the silane-depleted overhead fraction 11 produced from the light-endsdistillation column 5 may be treated to recover silane as describedabove. The overhead fraction 13 produced from the silane recoveryseparation unit 10 may be recovered by recycling the fraction to thelight-ends distillation column 5.

The heavy-ends distillation column 15, silane recovery separation unit10 and light-ends distillation column 5 may purify silane with smallamounts of silane being lost relative to conventional processes. Forexample and in accordance with some embodiments of the presentdisclosure, the sum of the amount of silane in the waste streams of theprocess of FIG. 6 (i.e., the overhead fraction 11 produced from thelight-ends distillation column 5 and the silane-depleted bottomsfraction 8 produced from the silane-recovery separation unit 10) is lessthan about 15 wt % and, in other embodiments, less than about 10 wt %,less than about 5 wt % or less than about 3 wt % of the silane in thesilane-containing stream 3.

Generally, in contrast to processes in which trichlorosilane is takenthrough a series of disproportionation and distillation steps to producea silane end-product, in embodiments for purifying silane, thecomponents of the feed stream 3 do not substantially undergo anyreaction until being discharged as the overhead fraction 11 producedfrom the light-ends distillation column 5, as the silane-enrichedoverhead fraction 43 produced from the heavy-ends distillation column15, the silane-enriched overhead fraction 13 produced from thesilane-recovery separation unit 10 or the silane-depleted bottomsfraction 8 produced from the silane-recovery separation unit 10.

In some embodiments, the feed stream 3 contains an amount of ethylene,most of which is separated into the bottoms fraction 7 produced from thelight-ends distillation column 5 and into the silane-enriched overheadfraction 43 produced from the light-ends distillation column 15.Referring now to FIG. 7, ethylene may be separated from silane byintroducing the overhead fraction 43 into an adsorber 52 to remove aportion of the ethylene (e.g., convert to another compound more readilyseparable from silane) and form an ethylene-depleted effluent stream 54as disclosed in U.S. Pat. Nos. 4,554,141; 5,206,004 and 5,211,931, eachof which is incorporated herein by reference for all relevant andconsistent purposes. The adsorber 52 may be a molecular sieve whichcauses a portion of the ethylene to react with silane and formethylsilane. The adsorber 52 may operate at a temperature from about 10°C. to about −100° C. or from about 0° C. to about 60° C. and at apressure of from about 1500 kPa to about 2600 kPa (about 220 psia toabout 380 psia) or from about 1700 kPa to about 2200 kPa (about 250 psiato about 320 psia).

The ethylene-depleted effluent stream 54 containing ethylsilane may beintroduced into an ethylsilane distillation column 60 to produce anethylsilane-depleted overhead fraction 62 comprising purified silaneproduct and an ethylsilane-enriched bottoms fraction 64 relative to theethylene-depleted effluent stream produced from the adsorber. Theethylsilane-enriched bottoms fraction 64 may be recycled by introducingthe ethylsilane-enriched bottoms fraction 64 into the light-endsdistillation column 5. The ethylsilane-depleted overhead fraction 62 maycontain less than about 10 ppm by weight ethylene and ethane or evenless than about 1 ppm, less than about 0.1 ppm or even less than about0.01 ppm ethylene and ethane. The ethylsilane distillation column 60 mayoperate at a temperature from about 10° C. to about −100° C. or fromabout 0° C. to about 60° C. and at a pressure of from about 1500 kPa toabout 2600 kPa (about 220 psia to about 380 psia) or from about 1700 kPato about 2200 kPa (about 250 psia to about 320 psia).

In certain other embodiments of the present disclosure, the light-endsdistillation column 5 may remove compounds with a boiling point lessthan silane after the silane-containing stream 7 is introduced into theheavy-ends distillation column 15. As shown in FIG. 8, thesilane-enriched overhead fraction 43 produced from the heavy-endsdistillation column 15 is introduced into a light-ends distillationcolumn 5′ to form a silane-depleted overhead fraction 11′ containingcompounds having a boiling point less than silane and a silane-enrichedbottoms fraction 6′ relative to the silane-enriched overhead fraction 43produced from the heavy-ends distillation column 15. In embodimentswherein the silane-containing stream 7 contains ethylene, the majorityof the ethylene may be separated into the silane-enriched bottomsfraction 6′ produced from the light-ends distillation column 5′. Thesilane-enriched bottoms fraction 6′ may be introduced into an adsorber(not shown) and ethylsilane distillation column (not shown) as in theprocess and system shown in FIG. 7 and described above. In this regardand as discussed above, the processes of FIG. 6 and FIG. 7 in whichlight-ends distillation 5 is performed before heavy-ends distillation 15may be preferred to the process of FIG. 8 in which heavy-endsdistillation 15 is performed prior to light-ends distillation 5′.

Processes for Purifying Silane that Include Recovery from BothLight-ends and Heavy-ends Streams

The processes described above (e.g., recovery from the light-ends streamand recovery from the heavy-end stream) may be operated in combinationto recover further silane from the waste streams. As show shown in FIG.9, a silane-containing stream 3 containing one or more compounds havinga boiling point less than silane and one or more compounds with aboiling point greater than silane is introduced into a light-endsdistillation column 5 to produce a silane-depleted overhead fraction 11and a silane-enriched bottoms fraction 6 relative to thesilane-containing stream 3. The silane-depleted overhead fraction 11includes silane and is enriched in compounds having a boiling point lessthan silane. The silane-depleted overhead fraction 11 produced from thelight-ends distillation column 5 is cooled in a condenser 37 to condensea portion of the silane therein. The silane-condensed overhead fraction42 is introduced into a gas-liquid separator 38 to produce a gaseousstream 35 and a liquid stream 51 containing condensed silane. The liquidstream 51 is thermally contacted with the silane-depleted overheadfraction 11 in a heat exchanger 36 (e.g., an interchanger) to condense aportion of the silane in the silane-depleted overhead fraction 11produced from the light-ends distillation column 5. The vaporized stream32 may be heated to about ambient temperature (e.g., in a heatingapparatus (not shown)) and introduced into the light-ends distillationcolumn 5.

The silane-enriched bottoms fraction 6 produced from the light-endsdistillation column 5 is introduced into a heavy-ends distillationcolumn 15 to produce a silane-enriched overhead fraction 43 and asilane-depleted bottoms fraction 31 relative to the silane-enrichedbottoms fraction 6 produced from the light-ends distillation column 5.The silane-depleted bottoms fraction 31 contains silane and is enrichedin one or more compounds having a boiling point greater than silanerelative to the silane-enriched bottoms fraction 6 produced from thelight-ends distillation column 5. The silane-depleted bottoms fraction31 produced from the heavy-ends distillation column 15 is introducedinto a silane-recovery separation unit 10 (e.g., distillation column) toproduce a silane-enriched overhead fraction 13 and a silane-depletedbottoms fraction 8 relative to the silane-depleted bottoms fraction 31produced from the heavy-ends distillation column 15.

In embodiments wherein the silane-containing stream 3 contains ethylene,the majority of the ethylene may be separated into the silane-enrichedoverhead fraction 43 produced from the heavy-ends distillation column15. The overhead fraction 43 may be introduced into an adsorber (notshown) and ethylsilane distillation column (not shown) as in the processand system shown in FIG. 7 and described above to separate ethylene fromsilane.

In this regard, it should be understood that the equipment shown in FIG.9 (e.g., light-end distillation, heavy-end distillation, condenser,silane-recovery unit and the like) may be operated as describedpreviously.

Generally, the components of the silane-containing stream 3 do notsubstantially undergo any reaction until being discharged as theoverhead fraction 11 produced from the light-ends distillation column 5,as the silane-enriched overhead fraction 43 produced from the heavy-endsdistillation column 15, or the silane-depleted bottoms fraction 8produced from the silane-recovery separation unit 10.

The process shown in FIG. 9 allows silane in the waste streams to berecovered. The amount of silane in the gaseous stream 35 separated fromthe condensed silane 51 may be less than about 10 wt % of the silane inthe silane-depleted overhead fraction produced from the light-endsdistillation column 5 or, as in other embodiments, less than about 8 wt%, less than about 6 wt %, or less than about 4 wt % of the silane inthe silane-depleted overhead fraction 11 produced from the light-endsdistillation column 5.

In these and in other embodiments, the amount of silane in thesilane-depleted bottoms fraction 8 produced from the silane-recoveryseparation unit 10 may be less than about 10 wt % of the silane in thesilane-depleted bottoms fraction 31 produced from the heavy-endsdistillation column 15, or, as in other embodiments, the amount ofsilane in the silane-depleted bottoms fraction 8 produced from thesilane-recovery separation unit 10 may be less than about 7.5 wt %, lessthan about 5 wt %, less than about 2.5 wt % or less than about 1% of thesilane in the silane-depleted bottoms fraction 31 produced from theheavy-ends distillation column 15.

In regard to the total amount of silane lost in the process and systemof FIG. 9, the sum of the amount of silane in the gas stream 35 and thesilane-depleted bottoms fraction 8 produced from the silane-recoveryseparation unit 10 may be less than about 10 wt % of the silane in thesilane-containing stream 3 or less than about 5 wt %, less than about 3wt %, less than about 1 wt %, less than about 0.5 wt % or even less thanabout 0.3 wt % of the silane in the silane-containing stream 3.

In this regard, it should be understood that the above-referencedprocesses may be operated in a parallel manner (e.g., more than onetrain of equipment that purify silane in a parallel fashion may be used)and flows from the various trains may be combined and/or process flowsmay be separated at any point within the process. Further in thisregard, it should be understood that the equipment described above(e.g., distillation column or separator) may include one or more unitsthat are operated in parallel or series (e.g., a distillation operationmay include use of two distillation columns operated in series orparallel) and reference to a single piece of equipment should not beconsidered in a limiting sense. It should also be understood that thesystems and processes shown in FIGS. 1-9 are exemplary and theillustrated systems and processes may be used in any combination withoutlimitation.

Systems for Purifying Silane from Light-ends Streams, Heavy-ends Streamsor Both Light-ends Streams and Heavy-ends Streams

The processes of the present disclosure may be carried out in a systemfor purifying silane such as, for example, the systems shown in FIGS.1-9 and described above. In some embodiments and as shown in FIG. 1, thesystem purifies a silane-containing stream 3 comprising silane and oneor more compounds having a boiling point less than silane. The systemincludes a light-ends distillation column 5 for producing asilane-depleted overhead fraction 11 and a silane-enriched bottomsfraction 6 relative to the silane-containing stream 3, thesilane-depleted overhead fraction 11 comprising silane and compoundshaving a boiling point less than silane. The system also includes acondenser 37 (FIG. 2) for condensing a portion of the silane in thesilane-depleted overhead fraction 11 produced from the light-endsdistillation column 5 and a gas-liquid separator 38 for producing agaseous stream 35 and a liquid stream 51 containing condensed silane.

The system also includes a conveying apparatus for conveying thesilane-depleted overhead fraction 11 produced from the light-endsdistillation column 5 (FIG. 1) to the condenser 37 (FIG. 2) and aconveying apparatus for conveying the silane-condensed overhead fraction42 to the gas-liquid separator 38. A heat exchanger 36 may be used tothermally contact the liquid stream 51 with the silane-depleted overheadfraction 11 produced from the light-ends distillation column 5 (FIG. 1)to condense a portion of the silane in the silane-depleted overheadfraction 11 produced from the light-ends distillation column 5. Aconveying apparatus conveys the silane-depleted overhead fraction 11produced from the light-ends distillation column 5 to the heat exchanger36 (FIG. 2) and a conveying apparatus conveys the partially condensedsilane-depleted overhead fraction 41 produced in the heat exchanger 36to the condenser 37. A conveying apparatus conveys the liquid stream 32or a vaporized product thereof into the light-ends distillation column 5(FIG. 1).

In some embodiments, the system (FIG. 3) includes a heavy-endsdistillation column 15 to produce a silane-enriched overhead fraction 43and a silane-depleted bottoms fraction 31 relative to thesilane-enriched bottoms fraction 6 produced from the light-endsdistillation column 5, the silane-depleted bottoms fraction beingenriched in compounds having a boiling point greater than silane. Aconveying apparatus conveys the silane-enriched bottoms fraction 6 fromthe light-ends distillation column 5 to the heavy-ends distillationcolumn 15. Alternatively and as shown in FIG. 4, the heavy-endsdistillation column 15′ may separate material prior to separation in thelight-ends distillation column 5 and a conveying apparatus conveys thesilane-enriched overhead fraction 3′ produced from the heavy-endsdistillation column 15′ to the light-ends distillation column 5.

In embodiments wherein the silane-containing stream 3 contains ethyleneand as further described below, the system may include an adsorber(e.g., molecular sieve) and an ethylsilane distillation column toproduce an ethylsilane-depleted overhead fraction comprising purifiedsilane product and an ethylsilane-enriched bottoms fraction relative tothe silane-enriched overhead fraction produced from the heavy-endsdistillation column.

In embodiments of the present disclosure in which crude silane containscompounds having a boiling point greater than silane and as shown inFIG. 5, the system includes a heavy-ends distillation column 15 forproducing a silane-enriched overhead fraction 43 and a silane-depletedbottoms fraction 31 relative to the silane-containing stream 7, thesilane-depleted bottoms fraction 31 comprising silane and being enrichedin one or more compounds having a boiling point greater than silanerelative to the silane-containing stream 7. The system also includes asilane-recovery separation unit 10 (e.g., distillation column) forproducing a silane-enriched overhead fraction 13 and a silane-depletedbottoms fraction 8 relative to the silane-depleted bottoms fraction 31produced from the heavy-ends distillation column 15. A conveyingapparatus conveys the silane-depleted bottoms fraction 31 produced fromthe heavy-ends distillation column 15 to the silane-recovery separationunit 10.

Referring now to FIG. 6, the system may also include a light-endsdistillation column 5 for producing a silane-depleted overhead fraction11 that is enriched in compounds having a boiling point less than silaneand a silane-enriched bottoms fraction 7 that is depleted in compoundshaving a boiling point less than silane relative to a silane-containingfeed stream 3 introduced into the light-ends distillation column 5. Aconveying apparatus conveys the silane-enriched bottoms fraction 7 tothe heavy-ends distillation column 15. A conveying apparatus conveys thesilane-enriched overhead fraction 13 produced from the silane-recoveryseparation unit 10 to the light-ends distillation column 5.Alternatively and as shown in FIG. 8, the heavy-ends distillation column15 may separate material prior to separation in the light-endsdistillation column 5′ and a conveying apparatus conveys thesilane-enriched overhead fraction 43 produced from the heavy-endsdistillation column 15 to the light-ends distillation column 5′.

Referring now to FIG. 7 and in embodiments wherein the silane-containingstream 3 contains ethylene, the system may include an adsorber 52 (e.g.,molecular sieve) for removing a portion of ethylene from thesilane-enriched overhead fraction 43 produced from the heavy-endsdistillation column 15 and to form an ethylene-depleted effluent stream54. A conveying apparatus conveys the silane-enriched overhead fraction43 produced from the heavy-ends distillation column 15 to the adsorber52. The adsorber 52 may be a molecular sieve which causes a portion ofthe ethylene to react with silane and form ethylsilane. The systemincludes an ethylsilane distillation column 60 and a conveying apparatusconveys the ethylene-depleted effluent stream 54 to the ethylsilanedistillation column 60. The ethylsilane distillation column 60 producesan ethylsilane-depleted overhead fraction 62 comprising purified silaneproduct and an ethylsilane-enriched bottoms fraction 64 relative to theethylene-depleted effluent stream 54 produced from the adsorber 52. Aconveying apparatus conveys the ethylsilane-enriched bottoms fraction 64into the light-ends distillation column 5.

In some embodiments and as shown in FIG. 9, the system may recoversilane from both the light-ends stream and the heavy-end stream inprocesses for purifying a silane-containing stream. As shown in FIG. 9,the system includes a light-ends distillation column 5 for producing asilane-depleted overhead fraction 11 and a silane-enriched bottomsfraction 6 relative to the silane-containing stream 3, thesilane-depleted overhead fraction 11 comprising silane and compoundshaving a boiling point less than silane. A condenser 37 condenses aportion of the silane in the silane-depleted overhead fraction 11produced from the light-ends distillation column 5. The system includesa gas-liquid separator 38 for producing a gaseous stream 35 and a liquidstream 51 containing condensed silane. A conveying apparatus conveys thesilane-condensed overhead fraction 42 from the condenser to thegas-liquid separator 38. The system includes a heat exchanger 36 forthermally contacting the liquid stream 51 with the silane-depletedoverhead fraction 11 produced from the light-ends distillation column 5to condense a portion of the silane in the silane-depleted overheadfraction 11 produced from the light-ends distillation column 5.

A conveying apparatus conveys the liquid stream 51 or a vaporizedproduct thereof to the light-ends distillation column 5. A conveyingapparatus conveys the silane-depleted overhead fraction 11 produced fromthe light-ends distillation column 5 to the heat exchanger 36. Aconveying apparatus conveys the partially condensed silane-depletedoverhead fraction 41 produced in the heat exchanger 36 to the condenser37.

The system includes a heavy-ends distillation column 15 for producing asilane-enriched overhead fraction 43 and a silane-depleted bottomsfraction 31 relative to the silane-enriched bottoms fraction 6 producedfrom the light-ends distillation column 5, the silane-depleted bottomsfraction 31 comprising silane and being enriched in one or morecompounds having a boiling point greater than silane relative to thesilane-enriched bottoms fraction 6. The system includes asilane-recovery separation unit 10 (e.g., distillation column) forproducing a silane-enriched overhead fraction 13 and a silane-depletedbottoms fraction 8 relative to the silane-depleted bottoms fraction 31produced from the heavy-ends distillation column 15.

In accordance with the system shown in FIG. 9, a conveying apparatusconveys the silane-depleted overhead fraction 11 produced from thelight-ends distillation column 5 to the condenser 37. A conveyingapparatus conveys the silane-enriched bottoms fraction 6 produced fromthe light-ends distillation column 5 to the heavy-ends distillationcolumn 15. A conveying apparatus conveys the silane-depleted bottomsfraction 31 produced from the heavy-ends distillation column 15 to thesilane-recovery separation unit 10. A conveying apparatus conveys thesilane-enriched overhead fraction 13 produced from the silane-recoveryseparation unit 10 to the light-ends distillation column 5.

In embodiments wherein the silane-containing stream 3 contains ethyleneand as further described above, the system may include an adsorber(e.g., molecular sieve) and an ethylsilane distillation column (both notshown) to remove ethylene from the overhead fraction 43 produced fromthe heavy-ends distillation column 15.

In this regard, suitable conveying apparatus for use in the systems ofFIGS. 1-9 are conventional and well known in the art. Suitable conveyingapparatus for the transfer of gases include, for example, arecirculation fan or blower. Suitable conveying apparatus for conveyingliquids include, for example, pumps and compressors. In this regard, itshould be understood that, use of the phrase “conveying apparatus”herein is not meant to imply direct transfer from one unit of the systemto another but rather only that the material is transferred from oneunit to another by any number of indirect transfer parts and/ormechanisms. For example, material from one unit may be conveyed tofurther processing units (e.g., purification) and then conveyed to thesecond unit. In this example, each unit of conveyance including theintermediate processing equipment itself may be considered to be the“conveying apparatus” and the phrase “conveying apparatus” should not beconsidered in a limiting sense.

Preferably, all equipment utilized in the systems for purifying silaneis resistant to corrosion in an environment that includes exposure tocompounds used and produced within the system. Suitable materials ofconstruction are conventional and well-known in the field of thedisclosure and include, for example, carbon steel, stainless steel,MONEL alloys, INCONEL alloys, HASTELLOY alloys, nickel, graphite (e.g.,extruded or iso-molded) and silicon carbide (e.g., converted graphite orextruded).

EXAMPLES Example 1 Modeling the Recovery of Silane from Light-endsDistillation Operations

Silane recovery from the overhead fraction of a light-ends distillationcolumn was modeled using ASPEN modeling software. The overhead fractionfrom light-ends distillation operations is introduced into aninterchanger where it was cooled to −94° C. (−137° F.) at 1825 kPa (250psig). The partially condensed overhead fraction containing condensedliquid (55 wt % of the feed) and vapor (45 wt % of the feed) isintroduced into a condenser that is cooled with liquid nitrogen. Thecondenser cools the partially condensed overhead fraction to −129° C.(−200° F.) at a pressure slightly less than 1825 kPa (250 psig). Thesilane-condensed overhead fraction contains liquid silane (77 wt % ofthe feed) that is 98 wt % pure and vapor (23 wt % of the feed). Theliquid silane stream may contain nitrogen (2 wt %), methane (0.5 wt %),hydrogen (0.06 wt %), ethane (0.004 wt %), and ethylene (0.001 wt %).

The condensed stream is introduced into a gas-liquid separatorcontaining a demister and operated at a pressure of slightly less than1825 kPa (250 psig) to produce a silane-depleted gaseous stream (23 wt %of the overhead fraction produced from the light-ends distillationcolumn) and a silane-enriched liquid stream (77 wt % of the overheadfraction produced from the light-ends distillation column) relative tothe silane-depleted overhead fraction produced from the light-endsdistillation column. The gaseous stream is warmed by a naturalconvection finned exchanger before neutralization and disposal. Theliquid stream (−129° C. (−200° F.)) is introduced into the interchangerto exchange heat with overhead fraction produced from the light-endsdistillation column to reduce energy requirements for the condenser. Theliquid stream is fully vaporized in the interchanger and is furtherwarmed in a convection finned exchanger. The vaporized liquid stream iscombined with fresh crude silane, compressed to 2170 kPa (300 psig) andintroduced into the light-ends distillation operations.

Example 2 Modeling the Recovery of Silane from Heavy-ends DistillationOperations

Silane recovery from the overhead fraction of a heavy-ends distillationcolumn was modeled using ASPEN modeling software. A system containinglight-ends distillation and heavy-ends distillation withoutsilane-recovery distillation was first modeled. The system includes twooperating trains in which crude silane (97.4 wt % silane with 0.8 wt %of compounds with a boiling point less than silane and 1.8 wt % ofcompounds with a boiling point greater than silane) is introduced into alight-ends distillation column. The light-ends distillation columns ofthe first and second trains each produce a silane-enriched bottomsfraction and a silane-depleted overhead fraction relative to crudesilane. The overhead fractions contained 3% of the silane in the feed.The bottoms fractions from the light-ends distillation columns were eachintroduced into a heavy-ends distillation column to produce asilane-enriched overhead fraction and a silane-depleted bottoms fractionrelative to the bottoms fraction produced in the light-ends distillationcolumn. The bottoms fractions produced from the heavy-ends distillationcolumn contained 3.4 wt % of the silane from the feed and the overheadfraction contained 96.6 wt % of the silane in the feed. The overheadfraction was at a temperature of −26° C. (−14° F.) and 2032 kPa (280psig). The silane yield was 93.8 wt % and 3.3 wt % of silane was lost inthe bottoms fraction of the heavy-ends distillation column.

The system described above was modeled with the bottoms fractionproduced from the heavy-ends distillation column of each train beingintroduced into a silane-recovery distillation column to produce asilane-enriched overhead fraction and a silane-depleted bottoms fractionrelative to the bottoms fraction produced from the heavy-endsdistillation column. The bottoms fraction produced from thesilane-recovery distillation column may be neutralized and disposed ofand the overhead fraction is recycled back to the light-endsdistillation columns. The bottoms fractions of the silane-recoverydistillation column is at a temperature of 36° C. (97° F.) and 1793 kPa(260 psig). In this system, the bottoms fractions produced from theheavy-ends distillation columns contain 3.7 wt % of the silane in thefeed and the overhead fractions produced from the heavy-end distillationcolumns (which may be used as silane product, optionally after furtherprocessing such as removal of ethylene) contain 96.3 wt % of the silanein the feed. The silane yield was 97.1 wt %, a 3 wt % increase from thesystem that did not include silane-recovery distillation columns.

Example 3 Commercial-Scale Recovery of Silane from Heavy-endsDistillation Operations

The commercial-scale system included two operating trains in which crudesilane was introduced into a light-ends distillation column. Thelight-ends distillation columns of the first and second trains eachproduced a silane-enriched bottoms fraction and a silane-depletedoverhead fraction relative to crude silane. The bottoms fractions fromthe light-ends distillation columns were each introduced into aheavy-ends distillation column to produce a silane-enriched overheadfraction and a silane-depleted bottoms fraction relative to the bottomsfraction produced in the light-ends distillation column. The bottomsfractions produced from the heavy-ends distillation column of each traincontained about 60 wt % silane and was introduced into a silane-recoverydistillation column at a temperature of 40° C. (104° F.) and pressure of2000 kPa (275 psig) to produce a silane-enriched overhead fraction and asilane-depleted bottoms fraction relative to the bottoms fractionproduced from the heavy-ends distillation column.

The silane-recovery column operated at an overhead pressure of 1320 kPa(177 psig) with a 2.2 kPa (0.32 psi) differential pressure across thecolumn. The bottoms temperature was 1.7° C. (35° F.). Thesilane-recovery column produced an overhead fraction with a silaneconcentration of 98.8 wt % and a bottoms fraction with a silaneconcentration of 7.7 wt % with the weight ratio of overhead fraction tobottoms fraction being 1.3:1. The overhead fraction was recycled to thelight-ends distillation column and the bottoms fraction was neutralizedand disposed of About 94% of silane was recovered by the silane-recoverydistillation column.

In this regard, it should be understood that the operating conditionsdescribed in Example 1-3 are exemplary and other conditions may be used.

When introducing elements of the present disclosure or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above apparatus and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying figures shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A process for purifying a silane-containingstream, the stream comprising silane and one or more compounds having aboiling point greater than silane and being substantially free oftrichlorosilane, tetrachlorosilane and tetraflourosilane, the processcomprising: introducing the silane-containing stream to a heavy-endsdistillation column to produce a silane-enriched overhead fraction and asilane-depleted bottoms fraction relative to the silane-containingstream, the silane-depleted bottoms fraction comprising silane and beingenriched in one or more compounds having a boiling point greater thansilane relative to the silane-containing stream; and introducing thesilane-depleted bottoms fraction to a silane-recovery separation unit toproduce a silane-enriched fraction and a silane-depleted fractionrelative to the silane-depleted bottoms fraction produced from theheavy-ends distillation column.
 2. The process as set forth in claim 1wherein the silane-recovery separation unit is a distillation columnwhich produces a silane-recovery separation unit silane-enrichedoverhead fraction and a silane-recovery separation unit silane-depletedbottoms fraction relative to the silane-depleted bottoms fractionproduced from the heavy-ends distillation column.
 3. The process as setforth in claim 1 comprising introducing a feed stream to a light-endsdistillation column to produce an overhead fraction and a bottomsfraction, the feed stream comprising silane, one or more compoundshaving a boiling point greater than silane and one or compounds having aboiling point less than silane, the overhead fraction being depleted insilane and enriched in compounds having a boiling point less thansilane, the bottoms fraction being enriched in silane and depleted incompounds having a boiling point less than silane, wherein a portion ofthe bottoms fraction forms the silane-containing stream that is fed tothe heavy-ends distillation column.
 4. The process as set forth in claim1 wherein the silane-enriched overhead fraction produced from theheavy-ends distillation column comprises compounds having a boilingpoint less than silane and wherein the silane-enriched overhead fractionis introduced into a light-ends distillation column to form asilane-depleted overhead fraction comprising compounds having a boilingpoint less than silane and a silane-enriched bottoms fraction relativeto the silane-enriched overhead fraction produced from the heavy-endsdistillation column.
 5. The process as set forth in claim 3 wherein thesilane-enriched fraction produced from the silane-recovery separationunit is recycled by introducing it into the light-ends distillationcolumn.
 6. The process as set forth in claim 3 wherein the amount ofsilane in the silane-depleted fraction produced from the silane-recoveryseparation unit is less than about 10 wt % of the silane in thesilane-depleted bottoms fraction produced from the heavy-endsdistillation column.
 7. The process as set forth in claim 3 wherein thesum of the amount of silane in the overhead fraction produced from thelight-ends distillation column and the silane-depleted fraction producedfrom the silane-recovery separation unit is less than about 15 wt % ofthe silane in the silane-containing stream.
 8. The process as set forthin claim 3 wherein the feed stream and the silane-enriched overheadfraction produced from the heavy-ends distillation column compriseethylene and wherein the silane-enriched overhead fraction produced fromthe heavy-ends distillation column is introduced into an adsorber toremove a portion of the ethylene and form an ethylene-depleted effluentstream.
 9. The process as set forth in claim 8 wherein the adsorbercomprises a molecular sieve which causes a portion of the ethylene toreact with silane and form ethylsilane, the process further comprisingintroducing the ethylene-depleted effluent stream which comprisesethylsilane to a ethylsilane distillation column to produce anethylsilane-depleted overhead fraction comprising purified silaneproduct and an ethylsilane-enriched bottoms fraction relative to theethylene-depleted effluent stream produced from the adsorber.
 10. Theprocess as set forth in claim 9 wherein the ethylsilane-enriched bottomsfraction is recycled by introducing the ethylsilane-enriched bottomsfraction into the light-ends distillation column.
 11. The process as setforth in claim 2 wherein the temperature of the silane-depleted bottomsfraction produced from the silane-recovery distillation column is fromabout 70° C. to about 130° C. upon discharge from the silane-recoverydistillation column.
 12. The process as set forth in claim 2 wherein thesilane-recovery distillation column operates at an overhead pressure offrom about 1500 kPa to about 2600 kPa (about 220 psia to about 380psia).
 13. The process as set forth in claim 3 wherein the components ofthe feed stream do not substantially undergo any reaction until beingdischarged as the overhead fraction produced from the light-endsdistillation column, as the silane-enriched overhead fraction producedfrom the heavy-ends distillation column, the silane-enriched overheadfraction produced from the silane-recovery separation unit and thesilane-depleted bottoms fraction produced from the silane-recoveryseparation unit.
 14. The process as set forth in claim 1 wherein thesilane-containing stream is substantially free of alkali or alkalineearth-metal silanes.
 15. A process for purifying a silane-containingstream, the stream comprising silane and one or more compounds having aboiling point greater than silane, the process comprising: introducingthe silane-containing stream to a heavy-ends distillation column toproduce a silane-enriched overhead fraction and a silane-depletedbottoms fraction relative to the silane-containing stream, thesilane-depleted bottoms fraction comprising silane and being enriched inone or more compounds having a boiling point greater than silanerelative to the silane-containing stream; and introducing thesilane-depleted bottoms fraction to a silane-recovery separation unit toproduce a silane-enriched fraction and a silane-depleted fractionrelative to the silane-depleted bottoms fraction produced from theheavy-ends distillation column; wherein the one or more compounds havinga boiling point greater than silane are selected from the groupconsisting of ethane, ethylene, ethyl-silane, diethyl silane, toluene,dimethoxyethane and mixtures thereof.
 16. A process for purifying asilane-containing stream, the stream comprising silane and one or morecompounds having a boiling point greater than silane, the processcomprising: introducing a feed stream to a light-ends distillationcolumn to produce an overhead fraction and a bottoms fraction, the feedstream comprising silane, one or more compounds having a boiling pointgreater than silane and one or compounds having a boiling point lessthan silane, the overhead fraction being depleted in silane and enrichedin compounds having a boiling point less than silane, the bottomsfraction being enriched in silane and depleted in compounds having aboiling point less than silane, wherein a portion of the bottomsfraction forms the silane-containing stream that is fed to theheavy-ends distillation column; introducing the silane-containing streamto a heavy-ends distillation column to produce a silane-enrichedoverhead fraction and a silane-depleted bottoms fraction relative to thesilane-containing stream, the silane-depleted bottoms fractioncomprising silane and being enriched in one or more compounds having aboiling point greater than silane relative to the silane-containingstream; and introducing the silane-depleted bottoms fraction to asilane-recovery separation unit to produce a silane-enriched fractionand a silane-depleted fraction relative to the silane-depleted bottomsfraction produced from the heavy-ends distillation column; wherein theone or more compounds having a boiling point less than silane areselected from the group consisting of hydrogen, nitrogen and methane.